A conventional SIC semiconductor device includes a metal-oxide semiconductor field-effect transistor (MOSFET) having a trench gate structure, which can increase a channel density, so that a large current can flow in the SIC semiconductor device. In the MOSFET made of SIC, electrical characteristics drastically change with a plane direction of a plane on which a channel is formed. Especially, an angle of a trench formed by etching influences a plane direction and electronic characteristics. Thus, an angle of a trench is important in a MOSFET having a trench gate structure. Ideally, it is preferred that a trench sidewall on which a channel is formed is close to a (11-20) plane or a (1-100) plane, that is, a plane perpendicular to a (0001) plane or (000-1) plane, which can increase a channel mobility (see JP-A-H9-199724, which corresponds to U.S. Pat. No. 5,744,826).
Etching vertically with high accuracy is necessary in order that the trench sidewall is the (11-20) plane or the (1-100) plane, which can increase a channel mobility. It is known that SIC is generally dry-etched by reactive ion etching (RIE) or inductively coupled plasma (ICP) using SF6, Cl2, CF4 as etching gas. Ideally, it is preferred that a trench etching is performed vertically from a wafer surface.
However, because SIC is chemically stable substance, it is difficult to etch SIC vertically with high accuracy. Although it is ideal that an angle of a trench sidewall is vertical to the (0001) plane of a wafer surface as shown in FIG. 6A, actually, a trench sidewall inclines with respect to a plane perpendicular to the (0001) plane as shown in FIG. 6B.
Thus, the trench sidewall inclines with respect to the (11-20) plane or the (1-100) plane, which can increase the channel mobility, and a high channel mobility may not be obtained. When an inclination angle of the trench sidewall with respect to the (11-20) plane or the (1-100) plane is small, a decrease in the channel mobility is small. However, when the inclination angle is greater than a predetermined value, the channel mobility decreases, and a desired channel mobility may not be obtained.